A push-pull amplifier is a typical power amplifying means. A push-pull amplifier is an amplifier made from a pair of transistors and is such that the output from each of the transistors in the pair is synthesized to obtain a signal output (for instance, refer to JP (Kokai) 2003-060,451 (pages 4 and 5, FIG. 1)).
A circuit diagram of a typical power amplifying apparatus comprising a push-pull amplifier is shown in FIG. 1. A power amplifying apparatus 10 in FIG. 1 comprises an input terminal X1, an operational amplifier A11, a push-pull amplifier 100, and an output terminal Y1. Input terminal X1 is connected to the inverting input of operational amplifier A11. The output signals of operational amplifier A11 are input to a push-pull amplifier 100. The output signals of push pull amplifier 100 are applied to output terminal Y1 and to the non-inverting input of operational amplifier A11. It should be noted that a load Z is connected to output terminal Y1 in order to facilitate understanding of the description that follows. Moreover, load Z is a resistor.
Push-pull amplifier 100 is a bipolar push-pull amplifier that uses MOS-type field effect transistors. Push-pull amplifier 100 comprises an input terminal S1, an output terminal T1, a P channel MOS-type field effect transistor Q11, and an N channel MOS-type field effect transistor Q12. The source of field effect transistor Q11 is connected to a positive power source Vcc. The source of field effect transistor Q12 is connected to a negative power source Vee. The output signals of operational amplifier A11 are applied through a constant-voltage source E11 to the gate of field effect transistor Q11. Moreover, the output signals of operational amplifier A11 are applied through a constant-voltage source E12 to the gate of field effect transistor Q12. The drain of field effect transistor Q11 and the drain of field effect transistor Q12 are connected together. Moreover, these drains are connected to output terminal Y1. In short, push-pull amplifier 100 is a drain output-type push-pull amplifier that outputs signals that have been inverted with respect to signals received at S1. Constant-voltage source E11 and constant-voltage source E12 are used for bias. Push-pull amplifier 100 can be operated as a class A, class AB, or class B amplifier by adjusting the magnitude of the bias.
Moreover, operational amplifier A11 acts in such a way that there is no difference between the voltage at input terminal X1 and the voltage at output terminal Y1. As a result, power amplifying apparatus 10 power-amplifies signals input from input terminal X1 and outputs them from output terminal Y1.
The loop gain (dB) of power amplifying apparatus 10 is the sum of the amplification factor (dB) of operational amplifier A11 and the amplification factor (dB) of push-pull amplifier 100. In general, the amplification factor of operational amplifier A11 is set so that it decreases with an increase in frequency, while the amplification factor of push-pull amplifier 100 is set so that it is constant regardless of frequency. Moreover, the amplification factor of push-pull amplifier 100 decreases as the drain current of field effect transistor Q11 or Q12 becomes smaller. The reason for this is as follows.
The amplification factor GM of push-pull amplifier 100 is the sum of amplification factor gm1 of field effect transistor Q11 and amplification factor gm2 of field effect transistor Q12. The AC amplification factor of a field effect transistor is represented by mutual conductance gm. It should be noted that mutual conductance gm is also represented as forward transmission admittance |Yfs|. The mutual conductance of a field effect transistor is virtually zero as long as the gate voltage is the threshold voltage or less. The drain current increases as gate voltage rises. As a result, mutual conductance gm increases. Therefore, it is assumed that push-pull amplifier 100 operates as a class AB amplifier. In general, the drain current of field effect transistor Q11 is large and the amplification factor gm1 of field effect transistor Q11 is also large when push-pull amplifier 100 outputs a large positive voltage. In this case, gm1 dominates the amplification factor GM; therefore, GM is large, as well as gm1. The drain current of field effect transistor Q11 decreases and gm1 also becomes smaller as the output voltage of push-pull amplifier 100 gradually decreases. The drain current of field effect transistor Q12 at this time is small and the amplification factor gm2 of field effect transistor Q12 is also small. There is a gradual increase in gm2 as the output voltage of push-pull amplifier 100 falls closer to zero. However, the amplification factor GM is quite small at this time. Thereafter, amplification factor gm2 of field effect transistor Q12 increases with a further reduction in the output voltage of push-pull amplifier 100. The amplification factor GM is now dominated by gm2 at this time; therefore, GM is large, as well as gm2. Similarly, the overall amplification factor GM changes dependent on the drain current of each field effect transistor and the output voltage of push-pull amplifier 100.
Power amplifying apparatus 10 has a finite frequency band; therefore, the frequency band of power amplifying apparatus 10 becomes narrower as the AC amplification factor of push-pull amplifier 100 decreases. Moreover, the frequency band of power amplifying apparatus 10 changes considerably as the change in the drain current of field effect transistor Q11 or Q12 becomes larger. Power amplifying apparatus 10 tends to vibrate and outputs an unwanted ringing waveshape as the amount of change in the frequency band increases. Furthermore, there is also a problem with this type of power amplifying apparatus 10 in that when there is high-speed change in the size of the load, it is difficult to maintain a constant output level.
The current amplification factor hfe of a bipolar transistor decreases with a reduction in the collector current. Consequently, the above-mentioned problem similarly occurs in a push-pull amplifier made from a pair of bipolar transistors and a push-pull amplifier made from a pair comprised of a bipolar transistor and a field effect transistor. Moreover, this problem becomes more obvious as the electrical efficiency of the push-pull amplifier improves. This is because the idle drain current and collector current are set at zero or at a relatively small value.
The present invention solves the above-mentioned problems, an object thereof being to provide a push-pull amplifier that prevents the reduction in the AC amplification factor that is attributed to changes in the collector current or the source current of the transistors. Moreover, an object of the present invention is to provide a power amplifying apparatus comprising this type of push-pull amplifier.